This invention concerns material removal systems and, more particularly, relates to apparatus and methods for determining the depth of excision generated by ablation, for determining the thickness of unablated material below the excision, and for providing automatic endpoint control of an ablation process. "Excision" is defined herein to include incisions and other cutting phenomena.
The utilization of lasers for material removal has become increasingly important for both commercial and industrial applications. In a variety of medical procedures, such as opthalmic surgery, a significant component of the procedure involves excision of a selected region or volume of tissue, in a minimally traumatic manner. One such technique is transverse excision ("T-excision") in which transverse cuts are made in the eye to modify a cylindrical curvature.
A critical aspect of the ablation procedure is measurement of excision depth, and determination of the endpoint of the excision procedure. In procedures such as radial keratotomy (RK), excisions may be as deep as 95% of the stroma. This depth must be precisely controlled to ensure predictable and reproducible results from the procedure, and to avoid damage to underlying material. In particular, in opthalmic surgical procedures such as T-excisions and RK, the depth of excision and the thickness of remaining tissue underlying the excision are parameters critical to the success of the procedure, and must be carefully controlled to minimize trauma and avoid perforation of the eye. Heretofore, there has been no reliable method or apparatus for accurately determining these dimensions.
It is known that when a focused energy device ablates material, the ablation products leave the material at high velocity. As a result of this event, an acoustic pulse is generated within the material. This acoustic pulse has a duration of comparable order of magnitude to that of the ablating laser pulse. The acoustic pulse propagates into the material, which may be, for example, a corneal structure, at the speed of sound, in a direction away from the ablation site where it is generated.
The detection of acoustic pulses resulting from laser ablation is discussed in Dyer, et al., "Nanosecond Acoustic Studies On Ultraviolet Laser Ablation Of Organic Polymers", Applied Physics Letters, Vol. 48, pp. 445-447, Feb. 10, 1986. This publication discusses the time evolution of ablative photodecomposition (APD), based on data generated by polyvinylidene fluoride (PVDF) piezoelectric transducers for sensing the acoustic pulses.
Moreover, it is known to provide acoustic feedback signals for controlling laser drilling. One such feedback loop is discussed in U.S. Pat. No. 4,504,727, Mar. 12, 1985, of Melcher, et al. The Melcher, et al. patent discloses the utilization of an acoustic sensor, such as a microphone or piezoelectric transducer, for sensing the initial pulses resulting from laser ablations. Endpoint detection is provided on the basis of differences in acoustic signal signatures between different layers, such as copper and epoxy-glass, in circuit boards.
Conventional systems such as those disclosed in these publications, however, cannot provide reliable measurement of the depth of excision or the remaining thickness of unablated material underneath the excision.
It is accordingly an object of the invention to provide methods and apparatus for determining the depth of excision generated by ablation.
It is another object of the invention to provide methods and apparatus for determining the thickness of unablated material below the excision.
It is a further object of the invention to provide methods and apparatus for endpoint control of an ablation process.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.